There are two methods commonly used for detecting flame failure in burner systems associated with furnaces. In general, such burner systems comprise a main burner and a pilot burner, the pilot burner being provided since it is an efficient method for igniting the fuel-air mixture from the main burner.
The first method is based on the use of an alloy rod (usually a high nickel, chromium, iron alloy) known as a "flame rod" that is inserted into the front end of the main burner and extends into the combustion space. A voltage supply (typically 120 volts A.C.) is applied to the rod and the electrical conductivity to the earth potential via the flame is measured Since the flame is capable of partially rectifying an alternating current, flame failure can be detected by the absence of rectification in the applied current between the flame rod and the earth potential.
There are several disadvantages associated with the use of flame rods and these may be summarized as follows:
(a) Flame rods are subject to oxidation and corrosion in the high temperature environment existing within the furnace. Such deterioration is accelerated by the fact that the flame rod must be positioned to extend into the high temperature region of the flame.
(b) Rectification measurements must be carried out accurately since electrical conductivity of the hot refractories between the flame rod and the earth generally is very significant. The extent of rectification is the component of a total signal which must be identified in order to positively identify that a flame connection exists in the high voltage circuit being monitored.
(c) In situations where a furnace comprises a number of relatively closely spaced burners it can be difficult to be certain that measurements relate to the burner near the location of the flame rod.
(d) A power supply is necessary to drive the measuring circuit and an electronic circuit capable of detecting the extent of rectification is required.
The second known method for detecting flame failure in burner systems in furnaces is based on the use of an optical device to sense the presence of a flame. An entry port or sighting hole is provided in the main burner cowl and is fitted with an optical device which focuses the light emanating from the flame. The light is focused onto a photosensitive element so that the wavelength in the blue to ultra-violet range is measured by filtering in order to detect light from the flame rather than from the incandescent contents of the furnace.
Light detection devices have the following limitations:
(a) The devices do not sense some flames satisfactorily (in particular those fed by natural gas and other relatively non-luminous combustion mixtures).
(b) The devices are difficult to align with the correct area of the flame.
(c) Often, it is necessary to turn off the pilot flame in order to ensure that the main burner flame is being sighted and therefore proved.
(d) Vibration of the furnace and related equipment often causes difficulties in proper aligning of the devices.
A third approach, in which an applied current is conducted via the principal burner flame and an auxiliary flame such as the pilot flame, is the basis of flame monitoring circuits disclosed in U.S. Pat. Nos. 2,003,624 to Bower and 2,903,052 to Aubert. The Bower patent describes an arrangement to which an electrode from the grid of a glow tube contacts the pilot flame, which in turn intersects the grounded main flame. Flame failure interrupts the circuit and results in de-energization of a relay coil. Aubert describes a monitoring arrangement in which an electrically isolated pilot burner conducts an applied emf via its flame, a main burner flame and an ignition pilot burner in a detection circuit.
The application of an external voltage to a flame relies on the associated electrical conductivity through the flame to keep the flame in a "proved state". However, flame fluttering due to varying flame positions and swirling in burner systems cause the measured conductivity--which is all that can be measured once an applied voltage is impressed onto the system--to fluctuate considerably. Significant delays e.g. 2 to 4 seconds, must be built into the detection/alarm circuits to avoid false alarms due to the conductivity temporarily falling below a particular threshold level, but such delays often represent the entry of a large quantity of unburnt fuel into the burner with the attendant high risk of explosion. Systems with an applied voltage are also susceptible to false alarms since corrosion of the pilot burner tips and of the flame rods ultimately increases the electrical resistance between the sensor and the flame. Buildups of carbon, ash and other materials interfere with optical methods and also deposit on tips and rods, thus lowering their sensitivity and rendering the measuring circuit unpredictable.
U.S. Pat. No. 3,302,685 to Ono proposes a flame detection arrangement based on the observations that the natural electrical phenomena associated with chemical reactions and temperature differences within a flame result in an electromotive force (emf) in the flame, and that this emf can be monitored, for example, by means of an isolated electrical conductor in contact with the flame to provide an indication of the condition of the flame. Ono's arrangement has the advantage that no high voltage source is required and entails detection of the flame condition with a simple voltmeter in a circuit including the flame and an electrode in contact with the flame. Electrode degradation is a problem with this proposal, and the method also suffers from the fact that conductivity is effectively being measured, necessitating, as before, a significant delay time to avoid serious flame-out recordals.